Glossary: Ecosystem

This chapter's terms in context

Chapter 24 works with five terms describing how energy and matter flow through a community of living organisms and their physical environment: ecosystem, producer, food chain, food web and decomposer. An ecosystem is a functional unit made up of a community of living organisms (the biotic component) interacting with each other and with the non-living physical factors of their environment (the abiotic component, such as light, temperature, water and soil), and the flow of energy and the cycling of nutrients within this unit are what make it a genuinely interconnected system rather than merely a list of species and physical conditions occupying the same space.

A producer is an organism, typically a green plant, alga or photosynthetic bacterium, that manufactures its own organic food from inorganic raw materials using an external energy source (usually sunlight, through photosynthesis), and it occupies the first position in every food chain because it is the ultimate entry point through which energy from outside the ecosystem (the sun) becomes available to every other organism in it. A food chain is a straight-line sequence showing the transfer of food energy from a producer through a series of organisms, each consuming the one before it, with arrows conventionally drawn to show the direction energy flows, from the organism being eaten to the organism eating it, a frequent exam error is drawing the arrow in the opposite direction, showing what an organism eats rather than the direction energy actually flows.

A food web is a more realistic representation than a single food chain, showing how multiple interconnected food chains combine within a real ecosystem, since most organisms in nature eat more than one type of food and are eaten by more than one type of predator; because of this interconnection, a change affecting one species in a food web, such as a disease reducing the population of one prey species, can ripple outward to affect several other species that are not directly connected to it in any single food chain, an effect examiners frequently test by asking a candidate to trace and explain an indirect consequence through a food web diagram rather than a straightforward food chain.

A decomposer is an organism, typically a bacterium or fungus, that breaks down dead organic matter (the remains of dead organisms and their waste products) into simpler inorganic substances, releasing nutrients such as nitrogen and mineral ions back into the soil or water where producers can absorb and reuse them; without decomposers, nutrients would remain permanently locked inside dead organic matter, and no ecosystem could sustain the continuous cycling of nutrients that keeps its producers supplied. A common exam error is confusing a decomposer with a consumer that simply eats dead matter (a detritivore or scavenger), when a decomposer specifically breaks matter down chemically at the molecular level, completing the return of nutrients to a form producers can directly absorb again.

Paper 2 questions on this chapter frequently supply an unfamiliar food web diagram and ask a candidate to predict the effect of removing one named organism on several other organisms at different points in the web, and full marks depend on tracing every connected pathway shown in the diagram rather than considering only the most obvious direct link.

A related exam angle asks a candidate to construct a pyramid of numbers or a pyramid of biomass for a given food chain, and to explain why the pyramid typically narrows towards its apex: energy is lost as heat at each stage through respiration, so less energy, and consequently less biomass and usually fewer organisms, is available to support each successive trophic level above a producer.